Its possible to enable SSL over http for Nagiso Core (Red Hat/CentOS). The Nagios WEB UI can be made secure by enabling https instead of plain http in Nagios. After you are done installing Nagios Core in Linux, you need to enable OpenSSL in Apache. First we need to install mod_ssl and openssl and then edit nagios.conf in Apache.

Scripting Nagios Custom checks is not that hard. Its a case of getting hold of group of commands, piping them and extracting the Status of a Service and then passing them as Variables with If THEN ELSE STATEMENTS, that in-turn informs Nagios the status of the Service being monitored.